The present invention concerns suspension bridges comprising an essentially flat main structure, the upper surface of which forms the roadway for the transport means crossing the bridge, and a suspension structure, formed of a plurality of catenary wires connected to end piers of the bridge and of a plurality of vertical stays for suspending the main flat bridge structure to the catenary wires.
It is known the these suspension bridges have vibration frequencies of their own; normally, with no wind, the basic flexural vibration frequency differs from the basic torsional vibration frequency, both being generally very low. Nevertheless, the action of side winds varies said typical vibration frequencies, particularly because--especially in bridges with large transversal dimensions and/or a wide span, for instance motorway bridges--the flat suspended structure behaves, when actually exposed to side winds, similarly to a wing surface, hence with a "lifting" effect which greatly varies from one moment to the next.
As wind increases its strength, the two aforespecified vibration frequencies tend to approach, up to the point of coinciding: in these circumstances, the structure is thus subjected to so-called "flutter" conditions, i.e. to flexural-torsional deformations which may even result dangerous for the stability of the whole structure. The wind speed causing these phenomena is called "flutter speed".
These flutter phenomena, and the problems connected thereto, are already taken into consideration when planning suspension bridge: in fact, in calculating the structure, one tries to make sure that its flutter speed is very high, or anyhow considerably higher than that determined by the highest wind speeds registered in the bridge area, so that the risk of flutter phenomena is extremely low or almost none.
Various expedients have been proposed for this purpose. In particular, according to a fairly widespread technique, oblique or transversal windbracing stays are provided, which transversally stiffen the bridge structure and are therefore apt to resist to any flexural and/or torsional deformations thereof. This technique has obviously the drawback of making the bridge structure considerably heavier, and it is anyhow difficult to apply to very long bridges.
Another known technique consists in constructing the surfaces, along which run the transport means, in the form of so-called "transparent roadway", i.e. formed of gratings which, leaving a free passage of air in the vertical direction, greatly reduce the lifting effect of the bridge wing structure, consequently preventing any flutter phenomena. Even this technique has however some limitations as, though it can always be applied to roadways crossed by trains or by technical service means, it is nevertheless unthinkable for roadways used by private means.